new mark of chaos

for a while now I've been trying to figure out what to paint my chaos army and I have decided black over white, Malal's colors and its because of what i read about him by chance.

I think it is stupid of gw to come out with 5 gods of chaos and then sit back down and say no theres 4... Since the allmighty gw gods wont get off their buts and make rules for the one god I like I am starting a trhead as to what I should use for my Mark of Malal Rules. I have been using the mark of tzeentch points cost but to change it up I'm thinking it should let them reroll their armor saves instead of a static +1 somewhere in the statline.

I am fine with not having a cult marine dedicated to him though I just use Chosen in my black legion

I would like to know the communities feedback as to what rules you think should be the Mark of Malal
or if you like my idea let me know. I am seriously considering putting it on my homerule board but I need feedback first and I need this rule to be fair.

Thanks ahead of time everyone and like I said get in on the discussion lol 4CHAOS!